Double metal cyanide (DMC) complexes were discovered more than forty years ago by researchers from the General Tyre and Rubber Company (U.S. Pat. Nos. 3,404,109; 3,427,256; 3,427,334; 3,941,849) and are well-known catalysts for the polymerization of alkylene oxides and the copolymerization of alkylene oxides with carbon dioxide.
DMC catalysts obtained in the presence of an alcohol (TBA) and polyether polyols as complexing agents have been found very efficient in the copolymerization of alkylene oxides with CO2.
The effect of the metal salt used in the preparation of the DMC catalyst on the efficacy of CO2 fixation was studied by Kim et al. (Catalysis Today 2006, 111, 292-296) in the copolymerization of various epoxides catalyzed by DMC complexes obtained using TBA and PTMEG as complexing agents.
The article Green Chemistry 2008, 10, 678-684 describes the microwave-induced copolymerization of cyclohexene oxide with CO2 in the presence of DMC catalysts having TBA and polyethers as complexing agents. The use of microwaves is reported to give a polycarbonate with a higher CO2 incorporation. However, the need of microwave irradiation makes this process not very attractive from an industrial viewpoint.
The article by Kim et al. in Catalysis Today 2009, 224, 181-191, studies the effect of several polyethers as complexing agents of the DMC catalyst on the copolymerization of cyclohexene oxide with CO2.
WO 2012/032028 discloses the copolymerizaton of alkylene oxides and carbon dioxide by means of DMC catalysts having an unsaturated alcohol as a complexing agent. These complexing agents are disclosed to improve the incorporation of carbon dioxide into the polymer.
WO 2011/089120 discloses a process for the catalytic copolymerization of alkylene oxides with carbon dioxide that leads to a high content of incorporated CO2 in the polycarbonate. In this process the DMC catalyst is activated by bringing portions of the alkylene oxide into contact with the catalytic system before the copolymerization reaction.
EP 2548908 discloses the preparation of polyether carbonate polyols from alkylene oxides and carbon dioxide with a double metal cyanide (DMC) catalyst, where the DMC catalyst comprises at least one complex forming components comprising polycarbonate diol, polyethercarbonate polyol, polyethylene glycoldiol or poly(tetramethylene etherdiol). In this process, the DMC catalyst is obtained by a process in which the washing step is carried on with an aqueous solution of an organic complex and at least one of the complex forming components mentioned above.
US 2013/123532 relates to a process for the preparation of polyether carbonate polyols from alkylene oxides and carbon dioxide by means of a double metal cyanide catalyst (DMC). The presence of a certain amount of an alkaline metal hydroxide, metal carbonate and/or metal oxide in the cyanide-free metal salt, the metal cyanide salt or both the mentioned salts used for the preparation of the DMC catalyst is disclosed to improve selectivity (that is, reduce the ratio cyclic carbonate/linear polyether carbonate) and increase the catalyst activity towards CO2. In this process, the DMC catalyst is obtained by a process in which the washing step is carried on with an aqueous solution of an organic complex ligand.
EP 2441788 discloses the production of polyether carbonate polyols from alkylene oxides and carbon dioxide by means of a double metal cyanide (DMC) catalyst, where the reaction is carried out in a tubular reactor.
US 2003/149323 discloses a method for the production of polyether carbonate polyols from alkylene oxides and carbon dioxide by means of a multimetal cyanide compound having a crystalline structure and a content of platelet-shaped particles of at least 30% by weight.
US 2013/0190462 relates to a process for the preparation of polyether carbonate polyols by catalytic copolymerization of carbon dioxide with alkylene oxides with the aid of double metal cyanide (DMC) catalysts and in the presence of metal salts.
Though the effect of different modifications within the catalytic system on the content of incorporated CO2 in the resulting polymer has been evaluated, the influence of the washing step during the preparation of the DMC catalyst has not been reported.
In spite of the different procedures for preparing polyether carbonate polyols disclosed in the prior art, improved processes are still needed. In particular, processes that lead to polyether carbonate polyols with a high content of carbon dioxide, even under mild reaction conditions, and/or improved selectivity of the linear to cyclic product are desirable.
WO 2012/156431 discloses DMC catalysts which are prepared by carrying out a first washing step with an aqueous solution not containing any complexing agent other than the polyether ligand. These catalysts were found highly active in the polymerization of alkylene oxides to obtain polyols. However, this document contains no indication that these catalysts can also be used in the copolymerization of alkylene oxides with carbon dioxide.